Understanding the ground state properties of neutron matter – matter composed entirely of neutrons – is crucial to answering central questions in both nuclear physics and astrophysics. For example, the structure of neutron stars, the interpretation of gravitational waves from neutron star mergers and the identification of astrophysical sites for heavy element nucleosynthesis all depend on the equation of state of neutron matter at high densities. Conversely, low-density neutron matter affects our understanding of neutron-rich nuclei and its similarity to cold Fermi atoms provides stringent tests of our theories of strongly interacting fermions.
We have introduced an auxiliary-field quantum Monte Carlo (AFQMC) method to study neutron matter at low to intermediate densities. This approach's key advantage is in employing the chiral effective field theory framework while avoiding the numerical sign problem by using an attractive, spin-independent effective Hamiltonian. Our research builds upon this approach's success to explore neutron matter's quasiparticle properties. To do this, we calculate slight differences in energy caused by the addition or subtraction of neutrons to the system, necessarily introducing a numerical sign problem. However, this sign problem is small enough to be amenable to reweighting methods. We present our calculation of the quasiparticle effective mass and pairing gap in neutron matter and discuss further work.
